1. Field of the Invention
The present invention relates to high mast lighting systems such as are used on highways and at toll plazas.
High mast lighting systems include a vertically elongated central mast or pole surrounded by an annular ring or other shaped platform on which a plurality of light fixtures are mounted. The annular ring is adapted to be raised to the top of the mast and supported there in a locked position during use. A drive mechanism is provided to lower the ring to the base of the mast in order to facilitate maintenance and lamp changes. Typically, the support rings in high mast lighting systems are raised and lowered by the drive mechanism through a plurality of cables secured to the ring and passing downwardly from pulleys on top of the mast into the ring to the drive mechanism.
High mast lighting systems can attain great height, of over 100 feet or more, with the result that they are subjected to severe stresses, particularly during heavy weather. Further, such high masts are not necessarily perfectly circular nor straight throughout their length. Some warping or bending may occur. Moreover, in cold weather ice may form on the mast. Although the support ring is typically secured from unintended movement when at its uppermost position on the mast, it is also imperative that the support ring not impact against the mast when it is raised and lowered between the upper and lower positions, whether it be due to high winds or warping or icing of the mast. It is also important that the ring ride smoothly along the mast while it is raised and lowered so as not to put undue stress on the cables and the driving mechanism.
2. Description of the Related Art
A variety of different types of centering devices for high mast lighting systems have been proposed.
For example, U.S. Pat. No. 4,025,782 describes providing the ring with a plurality of radially inwardly directed arms, each terminating in a roller. However, no spring biasing is described and it is not clear how effective this structure will be if the mast is warped or high winds interfere with the raising or lowering of the ring.
U.S. Pat. No. 4,661,894 discloses a stabilizing device for a high mast lighting system in which the weight of the support ring is converted into a stabilizing and centering force applied to a plurality of stabilizing arms. The tension in the cables pivot the arms toward and into engagement with the mast so as to maintain a constant biasing force exerted by the arms and its associated rollers against the mast. This, however, is a complicated structure.
U.S. Pat. Nos. 4,228,488 and 4,429,355 disclose a raising and lowering system for a high mast lighting system in which the ascent and descent of the ring is stabilized by a plurality of stabilizing rollers mounted on elongated axles. The elongated axles are apparently resilient. However, these axles do not surround the mast and it is possible that under high wind conditions the rollers can slide out of engagement with the mast so as to disrupt the ascent or descent of the ring.
U.S. Pat. No. 4,092,707 discloses a high mast lighting system provided with a plurality of stabilizer arms connected to the support ring by parallel links. Through a series of cables, pulleys and springs, the stabilizer arms are urged into engagement with the mast due to the weight of the ring. This, too, is a complicated structure.
U.S. Pat. No. 3,805,054 discloses a high mast lighting system in which rollers are mounted on telescopic arms that are spring biased into engagement with the mast. Should the hoist cable fail during movement of the lighting rack, the guides can slow descent considerably due to the pressure exerted by the compression spring.
U.S. Pat. No. 4,348,717 discloses a high mast lighting system in which the ring is centered by a plurality of U-shaped guide arms, each having a roller at its center for engaging the mast. Coil springs are arranged on the ends of the guide arms to torsionally urge the rollers toward the axis of the support ring and into contact with the surface of the mast. However, this is a complex and relatively heavy solution to the problem of centering the light ring. Further, it is difficult to pretension the torsion springs with a sufficient torque when mounting the ring on the mast.